NiPAm polymers, including homopolymers, copolymers and terpolymers or higher containing N-isopropyl acrylamide structural units, are temperature responsive polymers due to their Lower Critical Solution Temperature, or LCST. As used herein, the term "LCST" describes the temperature at which the polymer solution experiences a phase transition going from one phase (homogeneous solution) to a two-phase system (a polymer rich phase and a solvent rich phase) as the solution temperature increases. These materials remain relatively inert in warm water or synthetic urine (greater than about 28.degree. C.), but disperse quickly in cold water (less than about 25.degree. C.) with immediate loss in mechanical strength. It has been discovered that due to their inverse solubility, these temperature-triggered materials are particularly useful in water-dispersible products, most particularly flushable personal care products such as diapers, tampons, feminine pads, pantiliners, etc. As used herein, the term "water-dispersible product" means a product which, when exposed to a fluid at a temperature of approximately 22.degree. C. for approximately 2 minutes, dissolves or fragments into pieces all of which pass through a 20 mesh screen.
The conventional synthesis of NiPAm polymers requires the use of an expensive and not readily available monomer, N-isopropyl acrylamide. Temperature responsive, N-isopropyl acrylamide polymers and copolymers are known to be synthesized by free-radical polymerization of N-isopropyl acrylamide or copolymerization of N-isopropyl acrylamide with another monomer. For example, polymerization reactions of (N-isopropyl acrylamide) in benzene (JP 63117016) and ethyl acetate (JP 63117017) are known. Crosslinked poly(N-isopropyl acrylamide) hydrogels are prepared by radiation polymerization of N-isopropyl acrylamide as disclosed in Macromolecules, 26(26), 7386 (1993). Similar hydrogels are also synthesized by copolymerization of N-isopropyl acrylamide and methylene bisacrylamide under various conditions. (See Polym. Commun., 32(11), 322 (1991); J. Polym. Sci., Part A: Polym. Chem., 30(10), 2121 (1992).
A number of random, block or graft copolymers containing N-isopropyl acrylamide monomer units have been synthesized by various polymerization methods. For instance, a number of monomers including butyl methacrylate, N-isopropyl methacrylamide and dextran sulfate have been polymerized with N-isopropyl acrylamide to prepare random copolymers (J. Chem. Eng. Jpn., 26(1), 89 (1993); J. Biomater. Sci., Polym. Ed., 5(4), 371 (1994); JP 58174408; JP 58215413)). Hoffman et al. synthesized carboxyl terminated poly(N-isopropyl acrylamide) oligomers, which were then reacted with biopolymers to form thermo-reversible polymer-enzyme conjugates (J. Biomater. Sci., Polym. Ed., 4(5), 545 (1993)). Furthermore, poly(N-isopropyl acrylamide) macromonomers have been synthesized and subsequently polymerized with other vinyl monomers to yield temperature-responsive copolymers (Makromol. Chem., 194(2), 551 (1993)).
The synthesis of a number of polymers, which do not contain N-isopropyl acrylamide monomer units, particularly polymethacrylamides, by amidation of polymeric acyl chloride is described in Makromol. Chem., 194(2), 363 (1993). This article discloses that poly(acrylic acid) reacts in either aqueous or organic media with low molecular weight amines and polymeric amines to produce polyamides. For example, poly(acrylic acid) is reacted with perfluoroalkylamines to give polymers having a degree of amidation of 4-100% (JP 03243609). DE 3700518 discloses poly(acrylic acid) in aqueous solution treated with long chain alkyl amines to prepare partially amidated polymers. Amidation of poly(acrylic acid) is further disclosed in the following references: Yakhak Hoechi, 30(5), 232 (1986); JP 60079013; Polym. Bull., 13(3), 195 (1985); DE 3009235; DE 2533443; J. Polym. Sci., Polym. Chem. Ed., 13(12), 2859 (1975); Eur. J Biochem., 17(3), 561 (1970).
Poly(N-isopropyl acrylamide) (PNiPAm) exhibits a cloud point or inverse solubility property, i.e. the polymer is soluble in water below about 34.degree. C. and insoluble above that temperature. Poly(N-isopropyl acrylamide) (PNiPAm) has been cited in several patents where its temperature sensitive properties have been utilized. N-substituted polyacrylamides have been proposed for wound dressings with good absorbing characteristics and easy removal from the wound surface (JP 6233809). Further, flexible laminated films for control of light transmission have been disclosed with PNiPAm (JP 5177757). U.S. Pat. No. 5,509,913 describes the potential use of conventional PNiPAm for applications in flushable personal care products. Two related patents, JP 58174408 and JP 84042005, assigned to the Agency of Industrial Science and Technology, disclose copolymers of N-isopropyl acrylamide and N-isopropyl methacrylamide and the use of these copolymers as aqueous adhesives and coatings.
Other temperature-responsive polymers are described. For example, U.S. Pat. No. 5,509,913, mentioned above, covers a broad range of temperature and ion sensitive polymers. Stafford et al. describes the use of hydroxypropylcellulose (HPC) as a temperature sensitive binder in J. Pharm. Pharmacol. (1978), 30(1), 1. Also, a salt sensitive water soluble polyurethane binder for flushable nonwoven fabrics is disclosed in U.S. Pat. No. 4,002,171, issued to Taft. Further, a salt sensitive water soluble terpolymer for making flushable paper diapers, bandages and sanitary towels is disclosed in Japanese Patent No. JP 5125123 and U.S. Pat. No. 5,312,883 assigned to LION Corp.
What is needed in the art is an inexpensive and environmentally safe way to produce NiPAm polymers for use as binder materials and thermoformable articles that readily disperse in cold water, but remain structurally sound in warm water. Moreover, a method of producing NiPAm polymers having tailored cloud points (i.e., specific cloud points within a desired temperature range) is also needed.